1. Field of the Invention
The present invention falls in the area of heat treatment of certain products, in particular deep-drawn metal sheet.
2. Description of the Prior Art
Spatially shaped parts from metal sheet are typically produced by deep drawing. This procedure is characterized in that a sheet with a suitable easy-to-deform microstructure is shaped and drawn over the edge of a female drawing die by a male punch, while being held typically by a blank holder. This drawing process may be carried out by other methods as well, e.g. by pressure exerted by a medium from one side or by an electromagnetic force. The shape of the product is governed by the shape of the tools. This process causes the material to harden due to cold deformation.
The higher the yield strength of the input stock, the greater is the springback effect. The springback causes problems with production precision and repeatability. This is why a new hot drawing process was introduced recently, which includes cooling of the formed part between the tools. This process involves processing of metal sheet which has been heated to the austenite region, then drawn while in the austenitic condition and then, thanks to rapid heat transfer to the female drawing die, cooled between the tools in such a way that hardening-type, i.e. martensititic, microstructure is obtained in the formed part. This leads to smaller dimensional variation caused by the springback effect. In some cases, a procedure is used wherein a spatially shaped cold-drawn part is heated to austenitic condition and then hardened between the tools without applying deformation.
A typical material used in this application is steel, for example a steel identified by Euronorm steel standards as 23MnB5, the strength of which is about 1,500 MPa upon quenching. Increasing the strength further by changing its alloying normally causes problems in achieving the required elongation values.
The safety components in automotive industry, in particular require the highest possible product of strength and elongation values, for the components to be able to absorb as large as possible amount of the impact energy upon crash by deforming with high flow stress and without premature instability or fracture failure. These requirements are met in part through suitable engineering design of the components and the use of metal sheets with unequal thickness.
The weakness of the solutions used so far is that the combination of strength and elongation of the formed steel part is uniform throughout its volume, regardless of any local functional requirements for the part. Where the desired properties are achieved simply by changing the thickness of the metal sheet, the resulting component becomes heavy and difficult to manufacture.
Said disadvantages are removed by the present invention.